Chickenpox is caused by acute infection with varicella-zoster virus (VZV). The virus spreads throughout the body and enters cells of the nervous system. Latent infection occurs and the virus establishes itself in dorsal root and cranial nerve ganglia. The latent virus subsequently can reactivate and present as zoster (shingles). Researchers and pharmaceutical companies have developed chickenpox vaccines but the side effect of shingles due to the live virus establishing a latent infection is still of concern. The ability of a live virus vaccine to enter and maintain a latent infection phase therefore can compromise the safety of live viral vaccines. Any change to the virus that decreases the probability of establishing or maintaining a latent infection can bring significant public health benefits.
Live vaccines are very popular despite the possibility of latent infection. For example, the live attenuated VZV vaccine based on the “Oka virus” (see, U.S. Pat. No. 3,985,615) prevents chickenpox but the virus used in this vaccine can enter a latent infection phase in vaccinated individuals and later cause zoster (Sharrar et al. Vaccine 19:916 (2000), Wise et al. JAMA 284:1271 (2000)) The Oka virus is attenuated. However the reason for this attenuation and its significance to the latency problem is unknown.
During latency of VZV a limited repertoire of viral genes are expressed including open reading frames (ORFs) 4, 21, 29, 62, 63, and 66. ORF29 transcripts have been detected in human and rodent ganglia by in situ hybridization and reverse-transcription followed by PCR. (Cohrs et al, J. Vir. 74:11464 (2000); Kennedy et al., Virology 289:218 (2000). ORF29 encodes a 130 kDa protein that binds to single-stranded DNA and localizes predominantly to the nucleus of virus-infected cells in vitro (Kinchington et al, J. Virol. 62:802 (1988)). ORF29 contains a nuclear localization signal within amino acids 9 to 154 and transport to the nucleus requires Ran and karyopherins (Stallings et al., J. Virol. 79:10370 (2005)). While ORF29 protein is present in the nucleus of lytically infected cells, the protein is in the cytoplasm of neurons from human ganglia (Grinfeld et al, virus Genes 29:317 (2004); Lungu et al, PNAS 95:7080 (1998)). ORF29 protein localizes to the cytoplasm of guinea pig enteric ganglia neurons and in an astrocyte-like cell line (Chen et al, J. Med. Virology (Suppl. 1):S71 (2003); Stallings et al., J. Virol. 80:1497 (2006)). Treatment with a proteosome inhibitor or expression of HSV-1ICPO or VZV ORF61 results in translocation of ORF29 protein to the nucleus in both guinea pig enteric ganglia neurons and the astrocyte-like cell line.
ORF29 protein is secreted from VZV-infected fibroblasts and is endocytosed by human neurons in vitro (Annunziato et al., J. Virology 74:2005 (2000)). The protein is present in endothelial and epithelial cells in the skin of patients with varicella zoster; the protein is also located in nerves in the dermis of patients with varicella. ORF29 protein is not present in virions (Kinchington et al, J. Virology 66:359 (1992). The relationship of ORF 29 protein and latency has not been established.
Improved vaccines both for humans and for veterinary care, are needed that comprise altered viruses that present less risk of establishing or maintaining a latent infection and therefore are less likely to reactivate.